Application of Accurate Quantification Methods for Determining Emissions from the Acid Gas Removal Process in Natural Gas Processing

Abstract

Abstract This article presents new methods for accurately determining quantities of acid gas (CO2 and H2S) removed from acid gas removal (AGR) units in natural gas processing plants, and their applications to actual production data, with an emphasis on CO2 quantity determination. Greenhouse gas (GHG) emissions from anthropogenic activities contribute to climate change. The natural gas sector releases significant quantities of GHGs. AGR is a key step in natural gas processing, whereby H2S and CO2 are stripped from raw natural gas. Determining accurate CO2 emission quantities arising from AGR units has been challenging in the context of GHG emission quantification. The US Environmental Protection Agency and the Western Climate Initiative have each developed GHG quantification methods for petroleum and natural gas systems which include the AGR process. However, there is uncertainty about the accuracy of these approaches since not all aspects of the AGR process are taken into consideration. The proposed new methods for AGR GHG quantification are assessed using production data over three years from three natural gas plants. Assuming all other process factors are held constant, quantities of removed acid gas are functions of: •CO2 and H2S content in the inlet and outlet gas streams; and •the gas volume of either the inlet or outlet stream, depending on which quantification method is used. It is revealed that two main factors contribute to inaccurate GHG quantification from AGR units in practice so far: •the use of inlet gas stream volume instead of outlet gas stream volume; and, •failure to account for H2S content in the gas. In this study, inaccurately measured inlet gas stream volume was the primary cause of AGR CO2 quantification error and uncertainty. All calculation methods using inlet gas stream volume overestimated the CO2 quantity removed from an AGR unit by 3–11%. Quantification accuracy using volumes measured by inlet gas meters is limited because there is commonly a loss of gas volume from the inlet stream before it enters an AGR unit, and acid gas corrodes the metals in gas stream meters, which reduces meter accuracy. The H2S content of raw natural gas also has a significant impact on the accuracy of AGR CO2 removal quantification. Results show that methods using outlet gas stream volume without considering H2S content underestimate removed CO2 quantity by a factor of 1.1 times the H2S content in inlet gas stream. The error induced by this method linearly correlates to H2S content in the inlet gas stream. The higher the H2S content in the inlet stream, the larger the error. Calculation using outlet gas stream volume and H2S content in both inlet and outlet gas streams is recommended as a default method for quantifying GHG emissions from AGR units. Meanwhile, quantification methods for removed hydrogen sulfide (H2S) are also presented from the AGR process.